M.R.
The top agencies for children are Jet Set Models, Osbrink and LA Models. I see kids from these 3 agencies at every audition and on the call list for booked jobs. My son is with jet set & has booked Ralph Lauren and Children's Place print jobs. Every cent of his earnings go into his college fund.
Some advice - Run away from any agency that asks for any money from you. You should never have to pay for professional headshots since children change so much. Nor should any agent require that your child take acting lessons.
Also, it's a big commitment for not a lot of money if your child actually books a job. In addition, agents expect you to be ready to go to an audition at a moment's notice, so you always have to be "on call". And finally, if your child shows any signs that he/she is not happy or comfortable doing this, listen to him/her and immediately stop.
good luck.